The use of sucker rods within production tubing in an oil well is well known in the art. FIG. 1 is a schematic view illustrating a prior art pumping system. As shown in prior art FIG. 1, a pumping unit 1 is attached to a polish rod 2. The polish rod 2 is attached longitudinally to a sucker rod string 5 disposed inside of a tubing string 3 which is disposed in a casing string 4. The sucker rod string 5 is made up of multiple individual sucker rods according to American Petroleum Institute (“API”) 11B specification (hereinafter “API 11B sucker rod string”) coupled together in multiple connections 10 (only one shown). At the bottom end of the API 11B sucker rod string 5 is a reciprocating pump (not shown). As the pumping unit moves the sucker rod string 5 down, the barrel of the reciprocating pump fills with the production fluid 7 to be produced. Conversely, as the pumping unit moves the API 11B sucker rod string 5 up, a valve in the reciprocating pump shuts and the production fluid 7 in the pump barrel is lifted, displacing production fluid above it and forcing one pump-barrel's worth of production fluid 7 up the tubing string 3 in the annulus around the API 11B rod string 5 and the connections 10 on the way to the earth's surface and ultimately flowing out of the tubing string through valves and piping connections (not shown) and to storage and processing.
The API 11B sucker rod string 5 must extend from the pumping unit 1 all the way down to the reciprocating pump, which may be several thousand feet below the surface. As noted above, the API 11B sucker rod string 5 is made up of multiple individual API 11B sucker rods coupled together in multiple a connection 10. FIG. 1A illustrates an enlarged side cross section view of connection 10 of FIG. 1. The connection 10 includes a threaded pin end 29 on each of two adjacent API 11B sucker rods that are coupled together with a standard double female coupling 17. The coupling 17 is screwed onto the pins of the adjacent rods (“made up”) until the ends of the coupling contact a face 26 of a pin shoulder 28 of the sucker rod.
FIG. 2 is a side view cross-section illustrating a prior art API 11B sucker rod 20 having API specification 11B end connections on both ends. The sucker rod 20 includes a rod body portion 22 having a diameter DRAPI and an upset bead portion 23 that transitions from the rod body 22 to a wrench flat portion 27. The wrench flat portion 27 typically includes four wrench flats 24. The wrench flat portion 27 terminates in a pin shoulder 28 having a pin shoulder face 26 that contacts the end of the coupling 17 (see FIG. 1A) when the threaded pin ends 29 are made up in the coupling 17 to form the connection 10. The outer circumferential surface of the rod 20 includes a transition section that transitions from the rod body 22 to the bead 23. The transition section includes a radius of curvature RAAPI extending from the rod body diameter DRAPI to upset bead 23. The radius of curvature RAAPI terminates in an inflection point in the surface of the upset bead 23 wherein the surface of the upset bead then transitions to the wrench flat section 27. The wrench flat 24 has a width WSAPI (transverse to an axis AA of the rod 20) and a length LWSAPI (along the axis AA of the rod 20). The connection 10 further includes a pin length LPAPI and a stress relief length LSAPI. Table I below includes values for physical parameters expressed in millimeters (“mm”) and dimensionless ratios of the API 11B rod connection 10 as illustrated in FIG. 2.
TABLE ISucker Rod End Data for Prior Art API 11B Rod as shown in FIG. 2¾ inch⅞ inch1 inchNominal RodNominal RodNominal RodItemDiameterDiameterDiameterDRAPI (mm)19.0522.225.4RAAPI (mm)57.166.776.2LWSAPI (mm)31.7531.7538.1WSAPI (Minimum)24.724.732.6(mm)WSAPI (Maximum)26.126.134.1(mm)Range of1.3-1.371.11-1.181.28-1.34WSAPI/DRAPIWSAPI for FIG. 225.425.433.3embodiment (mm)WSAPI/DRAPI for FIG.1.331.141.312 embodiment (mm)RAAPI/DDAPI for FIG.2.9973.00532 embodiment (mm)
It has been observed that conventional API 11B sucker rods 20 have at least the following problems during use, e.g.:
Breakage in the zone of the upset bead 23 and wrench flat 24 due to fatigue failures (the upset bead is a zone frequently including a high number of surface defects) (due to the forging process that is used to produce that geometry) and the wrench square is subject to damage from torque wrenches when making up and breaking out the connection 20);
Many rods have to be reworked in the zone of the upset bead 23 due to kinking during forging; and
High wear of the rod string 5 due to erosion and corrosion by movement of production fluid 7 in the annulus between the inner wall of the tubing 3 and the sucker rod connections 10, 20.
A new sucker rod end connection design is desirable to overcome these problems and other problems with API sucker rod end connections.